1. Field of the Invention
The present invention relates to a process for preparing stereoisomerically enriched 4-aryl-4-hydroxybutanoic acid derivatives by reducing 4-aryl-4-ketobutanoic acid derivatives in the presence of ruthenium-containing catalysts.
2. Brief Description of the Prior Art
Stereoisomerically enriched 4-hydroxybutanoic acid derivatives are valuable intermediates, for example, in the preparation of liquid-crystalline compounds, agrochemicals and pharmaceuticals.
The catalytic reduction of ketones to stereoisomerically enriched secondary alcohols is known in principle. Useful reducing agents are customarily molecular hydrogen or, in the case of transfer hydrogenations, organic hydrogen donors, for example formic acid or isopropanol.
In EP A 744 401, Noyori et al. describe the enantioselective hydrogen hydrogenation of γ-keto esters and subsequent cyclization to the corresponding butyrolactones in the presence of chiral ruthenium complexes. A disadvantage is the extremely long reaction times of several days.
An advantage of transfer hydrogenations is that the safety measures which have to be taken when handling highly explosive molecular hydrogen under pressure are not required. In general, it is also possible to work at ambient pressure.
A review of transfer hydrogenations as a method for catalytically reducing ketones is given, for example, by Zassinovich et al. in Chem. Rev. 1992, 92, 1051-1069 and Noyori et al. in Acc. Chem. Res. 1997, 30, 97-102 and also Wills et al. in Tetrahedron, Asymmetry, 1999, 2045.
Noyori et al. (JACS 1996, 118, 2521-2522, Acc. Chem. Res. 1997, 30, 97-102) describe the use of ruthenium complexes as catalysts and triethylamine/formic acid as a hydrogen donor mixture for the enantioselective reduction of simple ketones.
However, there was also a need to provide an efficient process which allows the preparation of stereoisomerically enriched 4-aryl-4-hydroxybutanoic acid derivatives from 4-aryl-4-oxobutanoic acid derivatives.